The primary cause of change in regional metamorphism is pressure. Usually rocks subjected to high pressures at considerable depth within the earth's crust will exhibit regional metamorphism like the widespread phyllites, schists, gneisses and migmatites of the Canadian Precambrian Shield for example. A phyllite is the next higher metamorphic stage than a slate, a schist is a further stage and gneiss and migmatites are of the highest grade of regional metamorphism. On the other hand, contact metamorphism usually depends more on the effect of heat from an igneous intrusion say converting a limestone to a marble (more coarse grained than limestone usually).
